The functions of measuring and displaying vehicle speed, and measuring and displaying distance traveled, are now performed by well-known speedometer and odometer devices. These current devices have various shortcomings. For example, the odometer commonly consists of a series of drum-shaped dials appropriately geared together and driven by a flexible cable that is, in turn, driven by a pair of gears attached to the vehicle's drive train or driven by one of the vehicle's wheels. Operation is based upon the fact that the wheels nominally make a specific determinable number of revolutions per mile traveled and the drive train, being geared to the driving wheels, also makes a determinable number of revolutions per mile traveled. The odometer essentially counts wheel revolutions and, by means of gear ratios, converts wheel revolutions to distance traveled.
An inherent error arises from the fact that the gears used to connect the odometer drive cable to the vehicle's drive train must have an integral number of teeth. That is, gears cannot have fractional teeth, and for practical purposes the number of teeth on at least one of these gears is usually small. Thus, only a finite relatively small number of ratios is available, and it is rarely possible to match the gear ratio exactly to the odometer's requirement. A matching error of up to plus or minus four percent is considered acceptable under current practice. Further inaccuracies arise from the fact that normal variations in tire size, construction, materials, aging, inflation pressure, temperature, rotational speed, wear, load and tire-to-road interactions cause the actual number of revolutions a tire makes per mile traveled to vary from the nominal value. Greater accuracy and reduced mechanical complexity are needed.
The most common type of speedometer in current use utilizes a rotating magnet that is driven by the same flexible cable as the odometer, and that rotates inside a cup-shaped member made of a conductive material such as aluminum. Rotation of the magnet causes eddy currents in the conductive material which, in reacting with the magnet's rotating field, cause this member, which is rotatably mounted coaxially with the magnet, to try to rotate in the same direction as the magnet. However, rotation of the cup-shaped member is resisted by a spring, so that this member assumes an equilibrium position at which the force of the spring is balanced by the force of the reaction of the eddy current with the magnet's rotating field. Since the spring's rate is approximately linear and the eddy current reaction forces are approximately proportional to the magnet's rotational speed, which in turn is approximately proportional to vehicle speed, the rotational angle of the cup, measured from its neutral position, is approximately proportional to vehicle speed. A pointer attached to the cup member and rotating therewith points to numbers that are arranged on a dial to indicate this vehicle speed.
Since the speedometer is driven by the same cable as the odometer, it is subject to the same errors. However, by judicious placement of the numbers on the dial, the effects of gear ratio mismatch and variations in tire revolutions per mile at different speeds can be compensated. The other errors inherited from the odometer are not compensated, and the speedometer has three further sources of error of its own. First, both the speedometer spring, and the relationship between eddy current reaction force and the magnet's rotating speed, are slightly variable and nonlinear, making it impractical to achieve an accurate speed display over a wide range of speeds. Second, the mechanism is subject to hysteresis and backlash. Third, the output of the device is affected by temperature variations. Errors of three to four miles per hour due to the basic inaccuracy of the eddy current and spring mechanism, and of two to three miles per hour due to temperature variations, are considered acceptable. Another less common type of speedometer employs a small generator and an associated meter in place of the eddy current, spring and pointer arrangement, but this type suffers from the same error sources as the more common type speedometer. A more accurate and less complicated speedometer is therefore needed.
The functions of measuring and displaying tire inflation pressure during vehicle operation, and of warning the driver in the event of a dangerously underinflated tire, are not now performed by any existing device in widespread use on light vehicles. Several schemes have been developed for these purposes but, to date, all have been comparatively complex and expensive and, as a result, their use has been confined to specialized fields such as certain military vehicles. None has proved to be economical and reliable enough for widespread use on passenger cars and light trucks. However, these light vehicles do have need for such a system for two principal reasons. First, it is not unusual for a driver to be unaware of an underinflated tire, and to continue to operate a vehicle with this condition. This can damage or destroy the tire or, more seriously, cause an accident due to the changed handling characteristics imparted to the vehicle by a dangerously underinflated tire.
Second, to save cost, space and weight, many of the automobile manufacturers wish to eliminate the spare tire from their light vehicles--i.e., passenger cars and light trucks. To accommodate this goal, several tire manufacturers have developed so-called run-flat tires that can operate for a limited time at a limited speed--for example 50 miles at 50 miles per hour--when completely deflated. In theory, the driver of the vehicle equipped with such tires would not be stranded by an underinflated or flat tire, but would be able to continue driving for a reasonable distance to reach a service station and have the condition corrected. Thus, such a vehicle would not require a spare tire. However, this has not worked out in practice because the driver often is not aware of the change when a tire on his vehicle loses pressure or goes flat. Consequently, he continues to drive, and the result may be even worse than with a normal tire. Not only is the tire damaged or destroyed, but the driver and his passengers, having no spare tire, may be stranded upon the final complete failure of the tire. Because of this, many automotive engineers have concluded that the key to eliminating the spare tire from light vehicles, even those equipped with run-flat tires, lies in monitoring tire pressure and signaling the driver when pressure is dangerously low.
An object of this invention is to provide an economical and reliable system, method and apparatus for measuring the inflation pressures of pneumatic tires while they are operating on a vehicle and for displaying these pressures to the operator of the vehicle. Another object of this invention is to provide an improved device for measuring and displaying vehicle speed. A further object of this invention is to provide an improved device for measuring and displaying distance traveled by a vehicle. Yet another object of the this invention is to provide an economical and reliable mechanism for warning the operator of the vehicle if the pressure of a tire on the vehicle should fall below a preset threshold value. A further object of this invention is to provide a system, method and apparatus for monitoring tire inflation pressure during operation, and for sending a warning signal if a tire's pressure should fall below a preset threshold value, which continues its monitoring function after sending the warning signal and, if such continued monitoring indicates that the pressure is actually not below the threshold value, cancels the warning signal and resumes normal operation. Another object of this invention is to provide a system, method and apparatus as described above that can be calibrated according to the characteristics of the specific vehicle and service for which it is intended, either before the vehicle is put into service or later during its service life, or both. Another object of this invention is to provide a device as described above that can make use of certain systems or subsystems that are already present on many vehicles.